This invention relates to a device for automatically loading charges of sheet molding compound onto the lower die of a die set in a compression molding press.
Sheet molding compound (SMC) is a moldable composite material generally made up of an unsaturated liquid polyester resin, a low profile thermoplastic resin, an inert filler, magnesium oxide or other maturation aids, and short lengths (5 cm or shorter) of glass fiber reinforcement. The constitutents are mixed on a film of polyethylene or other such material. The mixture is covered with a second film. The SMC sandwich (film-SMC-film) is rolled-up and allowed to age until the polyester resin matures to a putty-like consistency. Preliminary to molding, the protective sheets are removed and the matured SMC is cut into pieces, generally strips, of appropriate size for a particular mold. These pieces are laid on the bottom mold of a matched set of top and bottom compression molds in a compression press. The molds are heated to cause flow and cure of the SMC. The molds are closed to make a part.
In the past, there has been no satisfactory way for automating the loading of SMC charges into a compression press. An operator had to lay cut pieces of SMC onto the hot lower mold. Manual charge loading is not only labor-intensive, but it can also cause poor and/or inconsistent properties in molded parts. For large parts, several strips or pieces must be loaded onto the lower mold. This can take several seconds. The pieces first laid on the hot mold begin to flow and cure before the press is closed. This may cause uneven cure across the part which can create deviations in mechanical properties and surface defects.
Therefore, there has been a long-felt need for developing a machine for automatically loading SMC in compression presses. However, several significant problems must be overcome.
SMC, by nature, is difficult to handle. Because of its putty-like consistency long strips need to be supported in at least two locations along their length to prevent excess sag as they are positioned in the mold. Attempts have been made to use barbed spears to pierce and then support SMC charges on the barbs. However, when these barbs are pulled back through the SMC, it leaves holes in the charges. Resin flows back into these holes as a part is molded in the compression press but the glass fiber does not. The resin-rich and fiberglass-poor locations are undesirable both mechanically and cosmetically.
Attempts have also been made to use a machine to grip the ends of long strips of SMC, pick them up and place them in the mold. However, the putty-like nature of the SMC makes it difficult to grab the edges without excessive sagging in the middle. Because the SMC is sticky, it is difficult to get it off the grippers.
Accordingly, it is an object of this invention to provide a novel device for automatically loading sheet molding compound charges into compression presses.